1. Field of the Invention
The invention relates in general to techniques for power switching regulators and in particular to regulators for space-based and ground-based power systems.
2. Description of the Prior Art
FIG. 1 illustrates a typical space power system, in which the labels R.sub.S and R.sub.L refer to the resistances of the source and reflected load comprised of electrical demands from circuit elements of a spacecraft, respectively. Energy from a solar array is regulated by a power conditioning unit and supplied to the load, while a battery supplies storage energy to meet demands during peak load periods or during periods of eclipse when the solar array is not supplying energy. Two common types of power conditioning units are shunt systems or peak power tracking (PPT) systems. In a shunt system, the array is tied directly to the load through a diode, and excess array power is dissipated by the shunt when the battery is fully charged. In a PPT system, the PPT continuously senses the operating point of the solar array and generates a control signal to maximize power from the array. The control signal is used to adjust the reflected load impedance (R.sub.L) through a switching converter to match the internal impedance of the array (R.sub.S).
The primary advantage of the PPT over the shunt system is that the power generated by the array may be maximized through control of the converter. This allows the array size to be reduced or the load increased for a particular mission. FIG. 2, which is a typical solar array characteristic curve, illustrates this advantage. To draw maximum power, it is necessary to match the load impedance to the internal impedance of the array. The PPT generates a control signal that selects the operating point. This signal is updated continuously since the characteristics of the array change due to variations in temperature, illumination intensity, radiation degradation, aging and partial failure. In contrast with a PPT, a shunt system must be biased far off the maximum power point to allow for these variations.
These aspects of a PPT are disclosed in the prior art. In U.S. Pat. No. 4,794,272, the phase of the output current is monitored as the operating point of the source is dithered. The phase of output current response with respect to the dither indicates which side of peak power point source is operating. However, the system becomes unstable when negative impedance loads are present. An example of a negative impedance load is a power converter input port since current decreases in response to a voltage rise, due to its constant power output.
E. N. Costogue and Dr. S. Lindera disclose a dynamic impedance comparison technique in "Comparison of Candidate Solar Array Maximum Power Utilization Approaches", 11th IECEC (1976). The operating point of the source is dithered while the resultant voltage and current are measured. Measured waveforms are multiplied and differentiated in order to determine the relationship of source impedance to load impedance. Negative feedback is used to drive the system to the operating point where the source and load impedances are equal, this point equating to maximum power. A significant shortcoming of this system is that it becomes unstable with negative impedance loads.
Another technique is a slope detection technique of Paulovich. "Slope Detection As A Method Of Determining The Peak Power Point Of Solar Arrays" NASA Report No. X-636-64-282 (Oct. 1964). A constant current dither is inputed into the source and the voltage response is measured in order to determine the slope of IV curve of the source. Negative feedback is used to drive the system to a predetermined source slope, which is expected to be near the maximum power point. However, changes in characteristics of the source complicate tracking and result in less than maximum power. Another shortcoming of this technique is that the system is potentially unstable with negative impedance loads.
Costogue also discloses a variation of dynamic impedance comparison technique which senses the onset of voltage collapse. Unlike the above references, this technique can be used to maintain stability while driving negative impedance loads. The relationship of source impedance to load impedance is first determined through mathematical manipulation of voltage and current in response to dithering of the source. Negative feedback is used to drive system to an operating point short of the maximum power point, thereby preventing source voltage collapse. The primary shortcoming of this technique is that only a fraction of maximum power is tracked.
A primary object of the invention is therefore a technique that supplies maximum power while maintaining stable operation with negative impedance loads, such as power supply inputs and arcjet engines. Another object of the invention is a simple PPT with a low parts count that results in low levels of overhead power, high reliability, and low production costs.